CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 23. Electrochemistry

potential can be measured. It is only when another half-cell is combined with the zinc half-cell that an electrical
potential difference, or voltage, can be measured.

The electrical potential of a cell results from a competition for electrons. In the zinc-copper voltaic cell described in
the previous lesson, the copper(II) ions were reduced to copper metal. That is because the Cu^2 +ions have a greater
attraction for electrons than the Zn^2 +ions in the other half-cell. Instead, the zinc metal is oxidized. Areduction
potentialmeasures the tendency of a given half-reaction to occur as a reduction in an electrochemical cell. In a
given voltaic cell, the half-cell that has the greater reduction potential is the one in which reduction will occur. In
the half-cell with the lower reduction potential, the reverse process (oxidation) will occur. Thecell potential (Ecell)
isthe difference in reduction potential between the two half-cells in an electrochemical cell.

Standard Cell Potentials

Thestandard cell potential (E^0 cell)isthe potential of an electrochemical cell when the temperature is 25°C, all
aqueous components are present at a concentration of 1 M, and all gases are at the standard pressure of 1 atm. The
standard cell potential can be calculated by finding the difference between the standard reduction potentials of the
two half-cells.

E^0 cell=E^0 red−E^0 oxid

Since the reduction potentials for half-cells cannot be measured independently, it is necessary to establish a standard
to serve as a reference. This reference is given a reduction potential of 0 volts by definition. Every other half-cell
can then be compared to this standard electrode in order to determine the reduction potential for any half-cell. The
standard hydrogen electrodeisa reference electrode that is used with another electrode (half-cell) to determine its
standard reduction potential. The standard hydrogen electrode (SHE) is shown below (Figure23.5).

The electrode itself is made of platinum, which serves as an inert surface upon which the oxidation or reduction
reaction takes place. The electrode is then placed in contact with both hydrogen gas (at a pressure of 1 atm) and an
acidic solution in which the concentration of H+is 1.0 M. Written as a reduction, the following half-reaction takes
place in a SHE:

2H+(aq)+2e−→H 2 (g) E^0 = 0 .00 V

Depending on the relative electrical potential of the other half-cell that the SHE is combined with, the hydrogen ions
may be reduced or the hydrogen gas may be oxidized. In general, reversing a reaction will also reverse the sign of
the corresponding electrical potential. However, reversing the above reaction has no effect on the standard potential
because the opposite of zero is still zero.

H 2 (g)→2H+(aq)+2e− E^0 = 0 .00 V

Determining Standard Reduction Potentials

When a standard hydrogen half-cell is connected to a standard copper half-cell and connected to a voltmeter (Figure
23.6 (A)), the reading is 0.34 V.

Observation of the cell shows that the copper(II) ion is reduced to copper metal, while the hydrogen gas is oxidized
to hydrogen ions. This is shown below along with the overall reaction taking place in the cell.